RFID transponder device and method for production of an RFID transponder device

ABSTRACT

An RFID transponder device has antenna terminals for coupling an antenna system to the device. A transmitter and a receiver are coupled to the antenna terminals. The device has at least one damping resistance connected to at least one of the antenna terminals. The at least one damping resistance is connected, depending on a voltage swing at the antenna terminals during a transmission burst period, either together with a serially connected switch in parallel to the antenna terminals that are coupled to the receiver, or together with a parallel connected switch between one of the antenna terminals and a terminal of the transmitter. A damping control is configured to activate the at least one damping resistance during a damping period after the transmission burst period by controlling the respective switch.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/397,559, filed on Jan. 3, 2017, which is a continuation of U.S.patent application Ser. No. 14/903,584, filed on Jan. 7, 2016, now U.S.Pat. No. 9,576,237, which is a national phase filing under section 371of International Patent Application No. PCT/EP2o14/062272, filed on Jun.12, 2014, which claims the priority of European Patent Application No.13175568.8, filed on Jul. 8, 2013, each of which applications are herebyincorporated herein by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to an RFID transponder device and to a method forproduction of such a device.

BACKGROUND

In various applications of inductive coupling communication, RFIDsystems are used which, for example, are operated at 13.56 MHz.Communication in these systems is done by magnetic coupling between anRFID reader coil and an RFID tag coil. RFID tags are passive devices,which for example are composed of an integrated circuit and some kind ofantenna that is attached to terminals of the integrated circuit. Forexample, data are transmitted from the tag to the reader using a loadmodulation. In other implementations, active transmission is used fortransmitting data. Examples for systems with active transmission can befound in WO 2013/002736 A1.

U.S. Patent Appl. Pub. No. 2010/0245039 A1 shows a low frequency wake-updevice with three receiver antennas, whose Q-factor can be adjusted, andwith a separate transmitter antenna. In contrast to RFID systems,different antennas and frequencies are used for transmitting andreceiving signals.

Active transmit systems need to synchronize their internal frequencysource to the frequency and phase of a carrier signal emitted by thereader. This is usually done using a phase-locked loop, PLL, systemwhich locks to the receive signal of the tag, in particular the inducedsignal resulting from the magnetic field of the reader. Accordingly, inconventional applications such a PLL is in a locked mode during areceive phase when the tag is not transmitting. When an activetransmission of the tag starts, the amplitude of the transmit signal, inparticular at the antenna of the tag, may be two orders of magnitudehigher than the amplitude of the receive signal. This can effect thatthe receive signal may be completely obscured by the transmissionsignal.

Accordingly, the PLL preferably is operated in an unlocked state or freerun state during this time. As it cannot be easily achieved that a freerunning PLL keeps its phase relation to a reader's carrier signal for alonger period of time, usually a resynchronization is performed inappropriate intervals. For example, the resynchronization is preferablyperformed in short time slots during a transmission of a data frame,when there is a pause in a transmit burst. However, due to the differentoscillation amplitudes between transmission and reception of signals,and in particular the need for lower oscillation amplitudes, it isdesirable to change the oscillation amplitude for resynchronization.

SUMMARY

The present disclosure provides an improved concept for RFID transpondersystems allowing a more accurate frequency resynchronization.

Various embodiments of an RFID transponder device for example compriseantenna terminals for coupling an antenna system to the device.Furthermore, such a device includes a transmitter coupled to the antennaterminals and configured to provide an oscillating signal to the antennaterminals, and a receiver coupled to the antenna terminals andconfigured to receive an oscillating signal at the antenna terminals.Such a device is particularly configured to transmit data by means ofthe transmitter in transmission burst periods. For example, such atransponder device is used in RFID tags using active transmission.

The improved concept is based on the idea that a voltage swing at theantenna terminals during a transmission burst period is too high fordetecting a carrier frequency signal, which can be used as a referencesignal for generating an internal clock signal. To this end, damping isperformed during a damping period after the transmission burst period.However, the kind of damping depends on an absolute value of the voltageswing during the transmission burst period. Hence, depending on thevoltage swing at the antenna terminals during the transmission burstperiods a damping resistance is connected either together with aserially connected switch in parallel to the antenna terminals that arecoupled to the receiver, or together with a parallel connected switchbetween one of the antenna terminals and a terminal of the transmitter.

A respective RFID transponder device therefore further comprises adamping control being configured to activate the at least one dampingresistance during the damping period by controlling the respectiveswitch. In particular, for the resistance with the serially connectedswitch, the switch is closed during a damping period, and for thedamping resistance with the parallel connected switch, the switch isopened during the damping period.

For example, for lower voltage swings during the transmission burstperiod the damping resistance in parallel to the antenna terminals canbe used. Preferably, a resistance value of the damping resistance isadapted to the antenna system coupled to the device, respectively theantenna terminals. In particular, the damping resistance value is in theorder of about 10 Ω to 100 Ω and is particularly higher than theresistance value of a closed switch taken alone.

For higher voltage swings at the antenna terminals, the dampingresistance connected serially between one of the antenna terminals and aterminal of the transmitter or the receiver can be used. Such anapplication is particularly useful if respective voltages are too highfor operating the integrated circuit respectively the device without therisk of damage or destruction of the device due to technologyspecifications.

In any case, the damping can be performed with a transponder deviceaccording to the improved concept such that resynchronization ispossible with an improved accuracy and within reasonably short time.

A transponder device according to the improved concept may comprise boththe serially connected damping resistance and the parallel connecteddamping resistance or just one of the respective damping resistances, ifthe voltage range of the voltage swing is known in advance. If bothkinds of structures are provided during production of such a transponderdevice, the non-used damping structure could be deactivated on atemporary basis or permanently. Accordingly, a transponder deviceaccording to the improved concept can be produced efficiently.

For example, an embodiment of an RFID transponder device according tothe improved concept further to the transmitter and the receivercomprises at least one damping resistance connected to at least one ofthe antenna terminals. The at least one damping resistance is connected,depending on a voltage swing at the antenna terminals during atransmission burst period, either together with a serially connectedswitch in parallel to the antenna terminals, or together with a parallelconnected between one of the antenna terminals and a terminal of thetransmitter or the receiver. The device further comprises a dampingcontrol being configured to activate the at least one damping resistanceduring a damping period after the transmission burst period bycontrolling the respective switch.

Preferably, the damping period consists of one to four carrier periodsof a carrier signal received from an RFID reader. For example, a timeperiod between two transmission burst periods consists of eight suchcarrier periods, as for instance defined in well known standards ISO/IEC14443 Type A or B.

A transponder device according to the improved concept may be configuredto synchronize to a carrier signal received at the antenna terminalsduring a synchronization period between the damping period and afollowing transmission burst period of the device during operation. Forexample, the synchronization period may consist of all or a part of theremaining carrier periods between the damping period and the followingtransmission burst period.

It should be noted that for various embodiments of the transponderdevice according to the improved concept, it can be implemented both asa single-ended and as a differential system. Both types of system,without damping, are well-known in the art. For example the transponderdevice may include a PLL circuit for generating an internal clockfrequency based on a signal at the antenna terminals after the dampingperiod, in particular between the damping period and the followingtransmission burst period. Such a PLL circuit may be located within thereceiver, the transmitter or may be combined with the receiver and thetransmitter on a common integrated circuit.

According to some embodiments, a first transmitter output terminal ofthe transmitter is connected to one of the antenna terminals by aparallel connection of a first damping resistance and a first switch. Inparticular, the first damping resistance is the at least one dampingresistance mentioned before. For example, such a configuration can beused with an antenna system that is connected by a single-endedconnection. This may also be combined with the application of an EMCfilter comprised by the antenna system. For example, the device maycomprise one or two additional terminals for connecting filtercapacitances of the antenna system. This additional terminal or theseadditional terminals may be coupled to a reference potential terminal byfurther damping resistances having a switch connected in parallel.

In particular for differential connections of the antenna system, asecond transmitter output terminal of the transmitter may be connectedto a further one of the antenna terminals by a parallel connection of asecond damping resistance and a second switch. Preferably, the first andthe second switch may be controlled concurrently by the damping control,i.e. with the same switching control signal.

In various embodiments of the transponder device, in addition or as analternative to the serially connected damping resistance, a seriesconnection of a fourth damping resistance and a fourth switch may beconnected between a first and a second receiver input terminal of thereceiver.

For example, if the transmitter antenna terminals are connected by thefirst and the second damping resistance with respective parallelconnected switches, and the antenna system comprises a balun, the fourthdamping resistance may be provided additionally.

In further embodiments the transponder device may contain a seriesconnection of the fourth damping resistance and the fourth switch andfurther comprises a bias circuit that is connected to the first and thesecond receiver input terminal of the receiver. The bias circuit isconfigured to provide a DC bias voltage to the antenna terminals. Hence,an oscillating signal at the antenna terminals, respectively thereceiver input terminals, oscillates around the bias voltage provided bythe bias circuit. This may have the effect that, depending on thevoltage swing and the bias voltage, the resulting voltage values becomepositive during a full oscillation, or in other words, no negativevoltages occur at the receiver input terminals.

For example, the bias circuit comprises a voltage source that isconnected to both the first and the second receiver input terminal by arespective resistive element.

The above example refers to a double-ended implementation with aparallel damping structure including the bias circuit and the seriesconnection of the fourth damping resistance and the fourth switch. Asimilar implementation can be made for a single-ended antennaconfiguration.

Accordingly, in some embodiments the transponder device may contain aseries connection of the fourth damping resistance and the fourth switchand further comprises a bias circuit. The bias circuit comprises avoltage source that is connected to a first receiver input terminal ofthe receiver by a resistive element and is configured to provide a DCbias voltage to the first receiver input terminal. In thisconfiguration, the first receiver input terminal is coupled to one ofthe antenna terminals, while a connection point of the voltage sourcewith the resistive element is coupled to a further one of the antennaterminals. The series connection of the fourth damping resistance andthe fourth switch is connected between the one and the further one ofthe antenna terminals described above.

For example, an antenna coil may be connected with one end to the firstreceiver input terminal, while the second end of the antenna coil isconnected to the connection point of the voltage source with theresistive element, respectively the further one of the antennaterminals. Also in this configuration an oscillating signal at thesingle receiver input terminal oscillates around the bias voltageprovided by the bias circuit. As described above, this may have theeffect that, depending on the voltage swing and the bias voltage, theresulting voltage values become positive during a full oscillation, suchthat no negative voltages occur at the receiver input terminal.

For example, the bias voltage in the embodiments above is selected onthe basis of the voltage swing, in particular to be at least as high asan amplitude of the voltage swing, or expressed differently, being abouthalf a peak-to-peak value of the voltage swing.

The decision whether to choose a parallel connected damping resistanceor a serially connected damping resistance between the antenna terminalsand the receiver or transmitter terminals is, for example, based on avoltage that the respective circuits of the transponder can handle. Inparticular, there usually are technology-dependent limitations thatassure that the circuits are not damaged or destroyed by too highvoltages. Hence, if an expected voltage swing at the antenna terminalsduring a transmission burst period is in an applicable range, theapproach with the fourth damping resistance connected between the firstand the second receiver input terminal in combination with the biascircuit may be used. In such applications the bias voltage for exampleis selected to be in the range of 40% to 60%, in particular 45% to 55%of a technology-dependent maximum voltage of the transponder device. Inother words, the bias voltage is around half the maximum voltage of thedevice. As a consequence, a voltage swing with an amplitude being safelyless than half that maximum voltage oscillates between a minimum voltagethat is still positive and a maximum voltage that is below thetechnology-dependent maximum voltage of the device.

In such and other embodiments, the transponder device, in particular thereceiver, may be free from an over-voltage protection with respect to apositive supply voltage. Accordingly, the respective device can be builtwith less effort.

In the various embodiments described above a resistance value of thefourth damping resistance may be selectable depending on characteristicsof the antenna system to be connected. In particular, an optimumresistance value can be determined with respect to capacitances andinductances of the antenna system. For example, higher resistances mayprovide a better damping for inductances, while lower resistance valuemay be more suitable for damping of oscillation with respect tocapacitances. An optimum value for the damping resistance may bedetermined by measurements or simulations in advance of finalizing thetransponder device. As mentioned before, such resistance value may be inthe range of 10 Ω to 100 Ω.

In various embodiments the transponder device comprises the fourthdamping resistance and the fourth switch as described before andfurthermore the bias circuit. The device additionally comprises theparallel connection of the first damping resistance and the firstswitch, the parallel connection being connected between one of theantenna terminals and the first transmitter output terminal of thetransmitter. This parallel connection forms a first damping structure,whereas the bias circuit and the series connection of the fourth dampingresistance and the fourth switch form a second damping structure. Insuch a configuration, the transponder device further comprises aselection unit configured to select one of the first and the seconddamping structure as being enabled and the other one of the first andthe second damping structure as being disabled. Preferably, thatselection is made on the basis of the voltage swing during thetransmission burst period, as described before.

In some embodiments, the selection may be made during operation of thetransponder device, for example depending on different operating states.In other embodiments, the selection unit is configured to perform theselection of the enabled damping structure once only.

For example, the transponder device generally is produced with bothvariants of the damping resistances. When the final application and therespective specifications for the transponder device are defined, thedamping structure, which is not needed according to the specifications,can be deactivated in a final production step. For example, theselection unit comprises one or more fuses that for instance bridge therespective damping resistances. Such fuses may be blown to activate ordeactivate, respectively, the damping resistances as needed.

As an alternative, the desired configuration may be stored in anon-volatile memory like an EEPROM. It is also possible to perform theconfiguration by programming through a host interface, if present.

Accordingly, in each case a single design for the transponder device canbe used that allows application for different system requirements.

For example, an embodiment of a method for production of an RFIDtransponder device according to the improved concept comprises provisionof antenna terminals for coupling an antenna system to the device. Atransmitter is provided that is coupled to the antenna terminals andconfigured to provide an oscillating signal to the antenna terminals. Areceiver is provided coupled to the antenna terminals and configured toreceive an oscillating signal at the antenna terminals. A first dampingstructure is provided that comprises a parallel connection of a firstdamping resistance and a first switch, the parallel connection beingconnected between one of the antenna terminals and a first transmitteroutput terminal of the transmitter. A second damping structure isprovided comprising a series connection of a further damping resistanceand a further switch being connected between a first and a secondreceiver input terminal. According to the improved concept, the methodcomprises selecting, in particular selecting once only, one of the firstand the second damping structure as being enabled and the other one ofthe first and the second damping structure as being disabled. Theselection depends on a voltage swing at the antenna terminals during atransmission burst period of the device during operation.

Further embodiments of the above-described method, in particular withrespect to further damping resistances, and, for instance, the biascircuit become apparent from the various embodiments of the RFIDtransponder device described above. For example, the first dampingstructure may be provided with a further parallel connection of a seconddamping resistance and a second switch, the further parallel connectionbeing connected between a second one of the antenna terminals and asecond transmitter output terminal of the transmitter.

Various implementations of the transponder device may be used,particularly in an RFID tag that preferably uses active transmission fortransmitting data from the RFID tag to an RFID reader.

BRIEF DESCRIPTION OF THE DRAWINGS

The text below explains the invention in detail using exemplaryembodiments with reference to the drawings. Components and circuitelements that are functionally identical or have the identical effectbear identical reference numbers. In so far as circuit parts orcomponents correspond to one or another function, description of themwill not be repeated in each of the following figures.

In the drawings:

FIG. 1 shows an exemplary embodiment of an RFID transponder device;

FIG. 2 shows a further exemplary embodiment of an RFID transponderdevice;

FIG. 3 shows an exemplary signal time diagram of signals within an RFIDtransponder device;

FIG. 4 shows a further exemplary embodiment of an RFID transponderdevice;

FIG. 5 shows a further exemplary embodiment of an RFID transponderdevice;

FIG. 6 shows a further signal time diagram of signals within an RFIDtransponder device;

FIG. 7 shows a further exemplary embodiment of an RFID transponderdevice;

FIG. 8 shows a further exemplary embodiment of an RFID transponderdevice;

FIG. 9 shows a further exemplary embodiment of an RFID transponderdevice;

FIG. 10 shows a further exemplary embodiment of an RFID transponderdevice; and

FIG. 11 shows a further exemplary signal time diagram of signals withinan RFID transponder device.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

FIG. 1 shows an exemplary embodiment of an RFID transponder device TRXthat is coupled to an antenna system AS. This and following embodimentsof a RFID transponder device, for example, are used in RFID tags or RFIDsystems for example operating at 13.56 MHz, which is also referred to ashigh frequency RFID. Communication in these systems is done by magneticcoupling between an RFID reader coil (not shown here) and an RFID tagcoil. RFID tags may be integrated in storage cards like micro SD cardsor SIM cards. For example, such RFID tags are operated with a powersupply provided and use active transmission instead of a load modulationused by passive RFID tags.

In the embodiment of FIG. 1, the RFID transponder device TRX comprises areceiver RX having receiver input terminals TR1, TR2, and a transmitterTX with transmitter output terminals TT1, 172. A first receiver inputterminal TR1 is coupled to a first input antenna terminal RFI1, whereasthe second receiver input terminal TR2 is coupled to a second inputantenna terminal RFI2. The coupling between the receiver input terminalsTR1, TR2 and the antenna input terminals RFI1, RFI2 may be a directconnection.

Similarly, a first transmitter output terminal TT1 is coupled to a firstantenna output terminal RFO1, whereas a second transmitter outputterminal TT2 is coupled to a second antenna output terminal RFO2. Inparticular, the first transmitter output terminal TT1 is coupled to thefirst antenna output terminal RFO1 by a first damping resistance RD1, towhich a first switch SW1 is connected in parallel. In a similar fashion,a second damping resistance RD2, to which a second switch SW2 isconnected in parallel, is connected between the second transmitteroutput terminal TT2 and the second antenna output terminal RFO2.

On the receiver side, a series connection of a further dampingresistance RD4 and a further switch SW4 is connected between the firstand the second antenna input terminal RFI1, RFI2. In other words, thedamping resistance RD4 with the serially connected switch SW4 form aparallel damping structure, whereas the serially connected resistancesRD1, RD2 with their respective switches SW1, SW2 form a serial dampingstructure.

The transponder device TRX comprises a selection unit SEL that isconfigured to control an operation or activation of the serial dampingstructure and/or the parallel damping structure.

The antenna system AS comprises an antenna coil LANT connected betweenthe antenna input terminals RFI1, RFI2 with a parallel connectedcapacitance CP. Furthermore, the parallel connection of the antenna coilLANT and the capacitance CP is connected to the antenna output terminalsRFO1, RFO2 by a respective capacitance CS1, CS2.

For example, the transponder device TRX is part of an active transmittag system, which needs to synchronize an internal frequency source to afrequency in phase of a carrier signal emitted by a corresponding readersystem. This may be done using a phase locked loop, PLL, system (notshown here), which may be a further part of the transponder device TRX.Such a PLL system may lock to the receiver signal of the tag thatcorresponds to a signal induced by the reader's system magnetic field.

During a transmission burst period an amplitude of a transmit signal onthe antenna system AS may be much higher than an amplitude of thereceive signal, to which the PLL tries to synchronize. For example, thetransmit signal is about two orders of magnitude higher than theamplitude of the receive signal. Accordingly, during the transmissionburst period, useful operation of the PLL may not be possible such thatthe PLL system stops synchronizing on the receive signal. However, asusually PLLs are not able to keep their phase relation for a longerperiod of time, a resynchronization is to be performed at short enoughintervals to keep a phase relation between the internal frequency sourceand the receive signal within desired boundaries. However, suchsynchronization can only be performed in the period between twotransmission burst periods. Furthermore, due to the higher amplitudeduring the transmission burst period and the oscillation properties ofthe antenna system, the amplitude on the antenna terminals decays slowlywithout application of specific measures. In particular, the transponderdevice TRX shown in FIG. 1 is able to perform a damping during a dampingperiod after the transmission burst period, such that synchronization ispossible.

Basically, the damping can be performed in two different ways. The firstkind of damping is performed by the parallel damping structure RD4, SW4with a closed switch SW4 during the damping period. Accordingly, energystored in the antenna system AS is reduced by the damping resistance,thus eliminating the high oscillation amplitude of the transmit signalsuch that the carrier signal can be received by the transponder deviceTRX, respectively the receiver RX and forwarded to the PLL system in asynchronization period following the damping period. For example, thedamping is controlled by the damping control, which may be part of anintegrated circuit comprising the transponder device TRX and which isnot shown here for reasons of a better overview only.

The second damping approach is performed with the serial dampingstructure with damping resistances RD1, RD2. Accordingly, during thedamping period the switches SW1, SW2 are open, whereas these switchesare in a closed state otherwise, bridging the damping resistances RD1,RD2. Also these damping resistances RD1, RD2 allow a reduction of theoscillation amplitude after the transmission burst period.

The selection unit SEL may activate one of the damping structures whiledeactivating the other damping structure, depending on a voltage swingexpected or measured during the transmission burst period. Hence, theRFID transponder device TRX can be adapted easily to variousapplications with higher or lower voltage swings during the transmissionburst periods. Such a selection may be performed reversibly duringoperation or before operation of the transponder device TRX, particularif different levels of voltage swings are to be expected. For example,the desired configuration may be stored in a non-volatile memory like anEEPROM. It is also possible to perform the configuration by programmingthrough a host interface, if present.

However, in alternative implementations, the selection may be made onceonly or irreversibly. Such a selection may be performed by the usage offuses or the like. The decision which of the damping structures is to beused may be made on the integrated circuit technology used for thetransponder device TRX, in particular, based on whether the technologyis able to withstand voltages at the antenna terminals without beingdamaged or destroyed.

FIG. 2 shows an implementation of the transponder device TRX, which isbased on the embodiment of FIG. 1. However, in the embodiment of FIG. 2,only the parallel damping structure is selected, whereas the serialdamping structure is omitted or deactivated, respectively. Additionally,a bias circuit BS is provided, which is connected to the receiver inputterminals TR1, TR2 and the antenna input terminals RFI1, RFI2,respectively. The bias circuit BS comprises a voltage source VDC, or arespective connection to a voltage source VDC, that is coupled to theterminals TR1, TR2 by respective resistors RDC1, RDC2. During operationof the transponder device TRX, in particular during the damping period,the bias circuit BS provides a bias voltage to the receiver inputterminals TR2, TR2, which forms a base level for the oscillating voltagesignal of the antenna system.

Preferably, the bias voltage is chosen such that it is at least as highas an amplitude of the voltage swing during the transmission burstperiod, or expressed differently, being about half a peak-to-peak valueof the voltage swing, such that no negative voltages result at thereceiver input terminals TR1, TR2. Preferably, both a lowest possiblevoltage value and a highest possible value at the antenna terminalsRFI1, RFI2 are both positive values and smaller than a voltage that canbe handled by the integrated circuit technology.

In preferable implementations, the bias voltage at the receiver inputterminals TR1, TR2 are set to about half of a technology-dependentmaximum voltage of the device. For example, the bias voltage is selectedto be in the range of 40% to 60%, in particular 45% to 55% of themaximum voltage. This can ensure that the amplitude does not decreasebelow a base level of the supply voltage. This also effects that aninput node ESD (electrostatic discharge) protection is not triggered onnegative amplitude peaks on the antenna. Such ESD clamping diodes may beinherently present between receiver input terminals TR1, TR2 and aground connection.

If a positive amplitude on the antenna terminals RFI1, RFI2 exceeds apositive supply voltage of the integrated circuit, respectively thetransponder device TRX, this can be handled using an input ESDprotection of the integrated circuit without clamping diodes to apositive supply terminal. Accordingly, the receiver RX may be free froman overvoltage protection with respect to a positive supply voltage.

In such a configuration, a limit in the positive direction is thus onlythe maximum voltage allowed by the integrated circuit technology.

Such configuration may ensure that a signal swing on the antennaterminals RFI1, RFI2 is inside predefined limits, which for example areset by a lower supply voltage and a higher supply voltage such that aconventional solid state switch can be employed and integrated into theintegrated circuit. Due to the fourth damping resistance RD4, theantenna system is not only short-circuited but results in a current flowthrough the resistance RD4, which effects energy dissipation of theenergy stored in the antenna system AS. In particular, the resistancevalue of the damping resistance RD4 may be adapted to the specificationsof the antenna system AS. Such a resistance value may be in the range of10 Ω to 100 Ω. Particularly, using the switch SW4 alone withoutemploying any additional resistance and thus only effecting thenegligible resistance of the closed switch, may not lead to any energydissipation depending on the variable switching times of such switchwith respect to the unknown carrier signal. For example, if a switchalone is closed at the time when the complete energy is stored in thecoil LANT, no energy will be dissipated.

For instance, an optimum resistance value of the fourth dampingresistance RD4 may be determined by means of calibration or simulationadvance of an actual operation of the transponder device TRX. Forexample, an optimum value can be found between a higher resistance valuebeing able to dissipate energy from the inductances and a lowerresistance for dissipating energy from capacitances.

FIG. 3 shows a signal time diagram of signals which may be present in asystem shown in FIG. 2. For example, the upper canvas 1 shows voltagevalues at both the antenna input terminals RFI1, RFI2. The second canvas2 shows a control signal which activates the damping structure duringits high state. As can be seen from the upper canvas, the signalamplitude of the signals at the input terminals RFI1, RFI2 can bedecreased efficiently. The two lower canvases 3, 4 show the respectivedriving signals at the antenna output terminals RFO1, RFO2. As can beeasily seen, the transmission burst period consists of eight consecutivecarrier periods of the corresponding reader system, as for instancedefined in well known standards ISO/IEC 14443 Type A or B. Accordingly,the damping period lasts about 2 to 3 of such carrier periods. Aresynchronization can be performed after the damping period in asynchronization period, which lasts up to the next transmission burstperiod. For example, an intermediate period between two transmissionburst periods lasts for at least eight carrier periods.

FIG. 4 shows a further implementation of the transponder device TRXwhich is based on the embodiment of FIG. 1. In particular, theembodiment of FIG. 4 can basically be seen as a single-ended version ofthe embodiment of FIG. 2. Hence, only the parallel damping structure isselected, whereas the serial damping structure is omitted ordeactivated, respectively.

The transponder device TRX of FIG. 4 comprises the receiver RX having asingle receiver input terminal TRI1, and the transmitter TX having asingle transmitter output terminal TT1. The receiver input terminal TR1is directly connected to a first antenna input terminal RFI1. A secondantenna input terminal RFI2 is connected to a voltage source VDC, whichis part of the bias circuit BS. The voltage source VDC is connected tothe receiver input terminal TR1, respectively the first antenna inputterminal RFI1 by means of a resistive element RDC1. The parallel dampingstructure RD4, SW4 is connected between the first and the second antennainput terminal RFI1, RFI2. The parallel connection of the antenna coilLANT and the capacitor CP is connected on one side to the first antennainput terminal RFI1 in a direct fashion, and connected to the antennaoutput terminal RFO1 by means of the capacitor CS1. The other end of theparallel connection of the antenna coil LANT and the capacitor CP isconnected to the second antenna input terminal RFI2.

Regarding preferable implementations of the embodiment of FIG. 4, it isreferred to the description of FIG. 2, in particular with respect to aselection of the bias voltage and a selection of an optimum resistancevalue of the fourth damping resistance RD4.

FIG. 5 shows a further embodiment of an RFID transponder device TRX withan antenna system AS coupled thereto. Also this embodiment is based onthe embodiment described in conjunction with FIG. 1. However, in theembodiment of FIG. 5 the parallel damping structure RD4, SW4 is omitted.In addition to the configuration of FIG. 1, the system shown in FIG. 5comprises additional capacitive elements CP1, CP2, which are connectedbetween respective ends antn, antp of the antenna coil LANT and a firstand a second auxiliary terminal CDMP1, CDMP2 of the transponder deviceTRX. These auxiliary terminals CDMP1, CDMP2 are respectively connectedto a reference potential terminal GND by means of respective dampingstructures being implemented as a parallel connection of dampingresistances RD3A, RD3B which respective switches SW3A, SW3B. Respectiveends antp, antn of the antenna coil LANT are connected to the antennainput terminals RFI1, RFI2 by means of the capacitive elements CIN1,CIN2.

Accordingly, in the embodiment of FIG. 5 a damping of the oscillationamplitude occurring during the transmission burst period is performed byserial damping structures. Such a decision or selection is, for example,made if the voltage swing of the signal on the antenna terminals is toohigh for the transponder device TRX or the corresponding integratedcircuit, respectively, to be directly connected.

Preferably, as shown in FIG. 5, a damping is implemented on nodes of theantenna system AS that do not exhibit voltage swings higher than asupply voltage of the transponder device TRX. To this end, the serialdamping resistances RD1, RD2, RD3A, RD3B are inserted in current pathsof the various antenna elements to dissipate the energy of the LC tankof the antenna system AS. As described in the previous embodiments, thedamping resistances RD1, RD2, RD3A, RD3B are activated during thedamping period by opening their respective switches SW1, SW2, SW3A,SW3B. Accordingly, an amplitude of the voltage swing can be reduced inappropriate time after each transmission burst period in order to make aresynchronization of the transponder device TRX, respectively its PLL,possible.

FIG. 6 shows an exemplary signal time diagram of signals that may bepresent in the embodiment of FIG. 5. Similar to the diagram of FIG. 3,the top canvas 1 shows signals on both antenna coil terminals antn,antp. The second canvas 2 shows the respective damping control signalthat opens the switches SW1, SW2, SW3A, SW3B during the damping period,thus having a low level during this damping period.

The third and the fourth canvases 3, 4 below depict both driving signalsat the antenna output terminals RFO1, RFO2, while the lower canvases 5,6 depict signals on the auxiliary terminals CDMP1, CDMP2.

As can be seen from the diagram of FIG. 6, the voltage swing at theantenna coil terminals antn, antp is reduced significantly during thedamping period, which follows the eight carrier cycles of thetransmission burst period, as described before for FIG. 3. Accordingly,a resynchronization on the carrier signal becomes possible in asynchronization period after the damping period.

While, with exception of FIG. 4, the above examples for transponderdevices according to the improved concept use differential signals,respectively terminals, for processing the antenna signals, the improvedconcept can also further be applied with single-ended systems.

For example, FIG. 7 shows a further exemplary embodiment of atransponder system with a transponder device TRX and an antenna systemthat is based on the embodiment of FIG. 1. For example, the embodimentof FIG. 7 can be seen as a single ended version of the embodiment ofFIG. 5. Hence, also in this embodiment no parallel damping structure butonly serial damping structures are present.

Accordingly, the receiver RX comprises a single receiver input terminalTR1, and the transmitter TX comprises a single transmitter outputterminal Tn. Accordingly, the serial damping structures include only thefirst damping resistance RD1 with the parallel connected switch SW1connected between the transmitter output terminal Tr1 and the antennaoutput terminal RFO1, and the parallel connection of the third dampingresistance RD3 and switch SW3 connected between the auxiliary terminalCDMP1 and the ground potential terminal GND.

The function of the embodiment of FIG. 7 becomes easily apparent fromthe description of FIG. 5 above.

FIG. 8 shows a further exemplary embodiment of a transponder system witha transponder device TRX and an antenna system that is based on theembodiment of FIG. 1. Similar to the embodiment of FIG. 5, also in thisembodiment no parallel damping structure but only serial dampingstructures are present.

With the single-ended concept, the receiver RX comprises only a singlereceiver input terminal TRI1, which is directly connected to a singleantenna input terminal RFI1. Accordingly, the transmitter TX comprises asingle transmitter output terminal TT1, which is coupled to the singleantenna output terminal RFO1 by the serial damping structure comprisingthe parallel connection of the damping resistance RD1 and the switchSW1. Like in the embodiment of FIG. 5, the transponder device TRXcomprises the auxiliary terminals CDMP1, CDMP2 which are connected tothe reference potential terminal GND by the serial damping structuresRD3A, SW3A and RD3B, SW3B.

The antenna system AS comprises the antenna coil LANT that is coupled tothe reference potential terminal GND on one side. The other side antp iscoupled to the antenna input terminal RFI1 by means of a capacitiveelement CIN1, to the first auxiliary terminal CDMP1 by a capacitiveelement CP1, and to the antenna output terminal RFO1 by a seriesconnection of capacitive elements CS1 and an EMC (electromagneticcompatibility) coil LEMC. A connection node of the capacitive elementCS1 and the coil LEMC is coupled to the second auxiliary terminal CDMP2by means of a capacitive element CEMC. The coil LEMC and the capacitiveelement CEMC together form an LC filter, which is used as an EMC filter,for example.

The EMC filter constitutes another LC system that stores energy duringthe transmission burst period. Hence, damping is also preferablyperformed on these elements LEMC, CEMC. Accordingly, the respectiveserial damping structures RM1, SW1, RD3B, SW3B are present to dissipateenergy from the EMC LC system during the damping period.

Accordingly, also with the embodiment of FIG. 8, damping of the voltageswing occurring during the transmission burst period can be achievedsuch that a resynchronization after the damping period is possible.

FIG. 9 shows a further embodiment of a transponder system with atransponder device TRX and an antenna system AS that at least partiallyis based on the embodiment of FIG. 1.

In the embodiment of FIG. 9, both serial damping structures and paralleldamping structures are used in the transponder device TRX. Furthermore,the antenna system AS comprises a single-ended antenna coil LANT that iscoupled on one side to a reference potential terminal GND. In order touse such a single-ended antenna with the differential inputs of thetransponder device TRX, the antenna system AS comprises a balun BL. Thesecond end of the antenna coil LANT is coupled to a first terminal ofthe balun BL, on the antenna side, by means of a capacitive element CS1and further to the auxiliary terminal CDMP1 by means of the capacitiveelement CP1. A second terminal of the balun BL on the antenna side iscoupled to the reference potential terminal GND. A third and a fourthterminal of the balun BL, being placed on the transmitter side, arecoupled to the antenna output terminals RF1, RFO2 by means of respectivecoils L1, L2. The antenna input terminals RFI1, RFI2 are also coupled tothe third and the fourth terminal of the balun BL. Furthermore, thethird and the fourth terminal of the balun BL are connected to thereference potential terminal GND by respective capacitive elements C3,C4.

During operation of the transponder device TRX, the balun is used toconvert the differential driver signal of the transmitter TX for thesingle-ended antenna use, as in this configuration the antenna coil ANTneeds a single sided signal. In such a configuration, the amplitude of asignal on the antenna may be too high to be directly connected to thetransponder device TRX, in particular the transmitter TX. Accordingly,in the embodiment of FIG. 9 the serial damping structures RM1, SW1, RD2,SW2 and further RD3, SW3 are present in order to dissipate energy of theantenna LC tank. Additionally, in order to dissipate the energy in thebalun BL, the parallel damping structure RD4, SW4 with a directconnection of the antenna input terminals RFI1, RFI2 to the differentialterminals, respectively third and fourth terminal of the balun BL isused. Preferably, the balun is designed such that it is operating on asufficiently low characteristic resistance that a voltage swing duringtransmission burst period is low enough to comply with the requirementsas described for the embodiment of FIG. 2.

In this case, the same result, in particular reducing of the voltageswing in a damping period, can be achieved, thus allowing theresynchronization after the damping period.

FIG. 10 shows a further embodiment of a transponder device TRX accordingto the improved concept, which is based on the embodiment of FIG. 1.Particularly, the embodiment of FIG. 10 can be seen as a double-endedversion of the embodiment of FIG. 8 employing an EMC filter, withfurther application of the parallel damping structure RD4, SW4 with thebias circuit BS.

In this embodiment, the single LC filter structure of FIG. 8, includingthe EMC coil LEMC and the EMC capacitor CEMC are replaced by tworespective EMC structures, respectively LC structures, consisting of thecombination of coil LEMC1 and capacitor CMC1, respectively coil LEMC2and capacitor CEMC2. The energy stored in these LC systems during thetransmission burst period can be dissipated by means of the serialdamping structures RD3A, SW3A, RD3B, SW3B at the auxiliary terminalsCDMP1, CDMP2 and by the damping structures RD1, SW1, RD2, SW2 at theantenna output terminals RFO1, RFO2.

In this embodiment, the serial damping can be combined with the paralleldamping at the receiver side, assuming that in the proposedconfiguration an amplitude of the signal on the antenna coil is notlarger than the maximum voltage allowed by the technology used toimplement the receiver RX.

Accordingly, also with the embodiment of FIG. 10, damping of the voltageswing occurring during the transmission burst period can be achievedsuch that a resynchronization after the damping period is possible.

FIG. 11 shows a further signal time diagram which acts as an example forsignals that may be present in a transponder device TRX according to theimproved concept, for example according to one of the embodimentsdescribed above.

The top canvas 1 depicts a signal on the RFID reader antenna, whereasthe canvas 2 below depicts an envelope of the signal of the top canvas.The third canvas 3 depicts a signal corresponding to a current of thetransmitter TX. In particular, respective transmission burst periodswith eight carrier cycles can be seen in the diagram of FIG. 11. Canvas4 below shows a signal on the antenna coil LANT of the tag comprisingthe transponder device TRX. The bottom canvas 5 depicts control signalsfor the damping of oscillations in the upper tray and resynchronizationin the lower tray.

It can easily be seen that in the signal of canvas 4, the oscillation isclearly reduced during the damping periods, allowing theresynchronization in the following resynchronization periods.

In the various embodiments described above, it becomes apparent thatdamping resistances may be provided and activated during damping periodsin order to reduce a voltage swing at terminals of the transponderdevice. Depending on an amplitude of the voltage swing, either paralleldamping structures or serial damping structures can be used andrespectively selected on the basis of the voltage swing. As describedabove, also a combination of serial and parallel damping structures canbe used.

Particularly, during production of such transponder devices TRX, thetransponder device TRX can be produced with both serial and paralleldamping structures, for example with all kinds of damping structuresdescribed above. When implementing the transponder device TRX, forexample in an RFID tag with known specifications, the respective dampingstructures that will not be needed during operation of the tag may bedeactivated, for example by a selection device SEL as described. Suchselection may be made permanently or once only, respectively. However,in some implementations such a selection could be possible also duringoperation of the transponder device TRX, for example if operatingconditions change during operation.

In any case, respective resistance values of the damping resistances canbe determined in advance based on calibration measurements orsimulations of the target system. In order to achieve a damping of thevoltage swing as fast as possible. For example, such damping periods maylast for two to four, in particular two or three carrier cycles of areader signal in order to have enough time during the resynchronizationperiod for a resynchronization on the carrier frequency.

What is claimed is:
 1. An RFID transponder device comprising: an antennaterminal configured to be coupled to a first terminal of an antenna; areceiver coupled to the antenna terminal; a transmitter coupled to theantenna terminal; a damping resistor coupled to the antenna terminal; aswitch coupled to the damping resistor; and a damping control circuitconfigured to activate the damping resistor by controlling the switchduring a damping period after a first transmission burst period of thetransmitter.
 2. The device of claim 1, wherein the damping controlcircuit is further configured to deactivated the damping resistor duringa synchronization period after the damping period and before a secondtransmission burst period by controlling the switch.
 3. The device ofclaim 1, further comprising the antenna.
 4. The device of claim 1,wherein the damping resistor comprises a resistance between 10 Ω and 100Ω.
 5. The device of claim 1, further comprising: a second antennaterminal configured to be coupled to a second terminal of the antenna;and a first resistor having a first terminal coupled to the receiver,wherein: the damping resistor is coupled in series with the switch, theswitch is coupled to a reference node, the reference node is coupled tothe first resistor and to the second antenna terminal, and thetransmitter is coupled to the antenna terminal via a first capacitor. 6.The device of claim 5, further comprising a bias circuit coupled to thereference node, the bias circuit configured to provide a bias voltage tothe reference node.
 7. The device of claim 1, further comprising: asecond damping resistor coupled between the transmitter and the antennaterminal, a first capacitor coupled between the second damping resistorand the antenna terminal, a second capacitor coupled between the dampingresistor and the antenna terminal, and a second switch coupled inparallel with the second damping resistor, wherein: the damping resistoris coupled in parallel with the switch, the damping resistor is coupledbetween the second capacitor and a reference node, the receiver iscoupled to the antenna terminal via a third capacitor, and the dampingcontrol circuit is further configured to activate the second dampingresistor by controlling the second switch during the damping periodafter the first transmission burst period.
 8. The device of claim 7,further comprising: a coil coupled between the second damping resistorand the first capacitor; a third damping resistor coupled between thereference node and the coil; a third switch coupled in parallel with thethird damping resistor; a fourth capacitor coupled between the thirddamping resistor and the coil; and the damping control circuit isfurther configured to activate the third damping resistor by controllingthe third switch during the damping period after the first transmissionburst period.
 9. The device of claim 1, further comprising: the antenna,wherein the antenna is coupled between a reference node and a balun viaa first capacitor; a second damping resistor coupled between a firstterminal of the transmitter and the antenna terminal; a second switchcoupled in parallel with the second damping resistor; a first coilcoupled between the second damping resistor and the antenna terminal; athird damping resistor coupled between a second terminal of thetransmitter and the antenna terminal; a third switch coupled in parallelwith the third damping resistor; a second coil coupled between thesecond damping resistor and the damping resistor; a fourth dampingresistor coupled between the reference node and the antenna; and afourth switch coupled in parallel with the fourth damping resistor,wherein: the antenna terminal is coupled to the first terminal of theantenna via the balun, the receiver comprises a first terminal and asecond terminal, the first terminal of the receiver being coupled to theantenna terminal, the switch is coupled between the first terminal ofthe receiver and the second terminal of the receiver, the switch beingcoupled in series with the damping resistor, and the damping controlcircuit is further configured to activate the second, third and fourthdamping resistors by controlling the second, third and fourth switchesduring the damping period after the first transmission burst period. 10.The device of claim 1, further comprising: a second antenna terminalconfigured to be coupled to a second terminal of the antenna; a firstresistor coupled between the antenna terminal and a first referencenode; a second resistor coupled between the second antenna terminal andthe first reference node; a first capacitor coupled between a firstterminal of the transmitter and the antenna terminal; a first coilcoupled between the first capacitor and the first terminal of thetransmitter; a second damping resistor coupled between the first coiland the first terminal of the transmitter; a second switch coupled inparallel with the second damping resistor; a second capacitor coupledbetween a second terminal of the transmitter and the second antennaterminal; a second coil coupled between the second capacitor and thesecond terminal of the transmitter; a third damping resistor coupledbetween the second coil and the second terminal of the transmitter; athird switch coupled in parallel with the third damping resistor; athird capacitor coupled between a second reference node and the firstcoil; a fourth damping resistor coupled between the second referencenode and the third capacitor; a fourth switch coupled in parallel withthe fourth damping resistor; a fourth capacitor coupled between thesecond reference node and the second coil; a fifth damping resistorcoupled between the second reference node and the fourth capacitor; anda fifth switch coupled in parallel with the fifth damping resistor,wherein: the receiver comprises a first terminal coupled to the antennaterminal and a second terminal coupled to the second antenna terminal,the switch is coupled between the first terminal of the receiver and thesecond terminal of the receiver, the switch being coupled in series withthe damping resistor, and the damping control circuit is furtherconfigured to activate the second, third, fourth and fifth dampingresistors by controlling the second, third, fourth and fifth switchesduring the damping period after the first transmission burst period. 11.A method of operating a transponder device, the method comprising:transmitting a signal to an antenna during a first transmission burstperiod; after the first transmission burst period, damping oscillationacross terminals of the antenna during a damping period by activating adamping resistor coupled to the antenna, wherein activating the dampingresistor comprises controlling a switch coupled to the damping resistor;after the damping period, deactivating the damping resistor andsynchronizing the transponder device; and after synchronizing thetransponder device, transmitting a signal to the antenna during a secondtransmission burst period.
 12. The method of claim 11, wherein the firsttransmission burst period comprises eight consecutive carrier periods.13. The method of claim 11, wherein the damping period lasts between 1to 4 carrier periods.
 14. The method of claim 11, wherein transmittingthe signal complies with an ISO/IEC 14443 Type A or B standard.
 15. Themethod of claim 11, further comprising determining a value of aresistance of the damping resistor during a calibration step.
 16. Themethod of claim 11, further comprising determining whether to couple thedamping resistor with the switch in series or in parallel based on avoltage swing across terminals of the antenna during the firsttransmission burst period.
 17. The method of claim 11, furthercomprising providing a bias voltage across terminals of the antenna. 18.The method of claim 17, wherein the bias voltage is about half apeak-to-peak voltage of a voltage swing across terminals of the antenna.19. The method of claim 11, further comprising storing a desiredconfiguration of damping resistor of the transponder device innon-volatile memory.
 20. A transponder device comprising: means fortransmitting a signal to an antenna during a first transmission burstperiod; means for damping oscillation across terminals of the antennaduring a damping period after the first transmission burst period byactivating a damping means coupled to the antenna; means fordeactivating the damping means and synchronizing the transponder deviceafter the damping period; and means for transmitting a signal to theantenna during a second transmission burst period after synchronizingthe transponder device.